1. Field of the Invention
The present invention relates to simultaneous interpenetrating polymer networks consisting of geopolymer and epoxy (hereafter referred to as SIN-GE), coatings utilizing SIN-GE compositions, and methods of making and applying compositions. A SIN is an interpenetrating polymer network obtained by the simultaneous crosslinking of two different polymer systems, without covalent bonds between the two networks [8].
2. Background
“Geopolymers” have been in use, under that name, since the 1970s, though the use of similar material occurred before that point. The term “geopolymer” refers to a class of aluminum silicate inorganic polymers. Geopolymer binders and cements are typically formed by reacting aluminum and silicon sources that contain AlO4− and SiO4 tetrahedral units under highly alkaline conditions at ambient temperatures. Metakaolin is a common aluminosilicate starting material in the formation of geopolymers, useful for manufacturing consistent geopolymers with predictable physical properties. Other aluminosilicate sources, such as Type F fly ash, have also been used.
Geopolymers typically have the following general formula [3]:Mn[—(Si—O2)z—(Al—O2)—]n Where M is a monovalent cation, z defines the ratio of Si to Al, and n is the degree of polymerization. M is typically an alkali metal such as lithium, sodium, potassium, cesium.
The ratio of Si to Al in a geopolymer defines the properties of a geopolymer and, therefore, also the possible applications of the geopolymer [6]. Geopolymers having a Si:Al ratio of 1:1 are known as poly(sialate) geopolymers. Those geopolymers having a Si:Al ratio of 2:1 are known as poly(sialate-siloxo) geopolymers. Those having a Si:Al ratio of 3:1 are known as poly(sialate-disiloxo) geopolymers. Typically, all of these types of geopolymers form three-dimensional networks that are very rigid. Higher ratios of Si:Al yield two-dimensional or even linear structures
Geopolymers are typically formed by mixing waterglass with a metakaolin (calcined aluminosilicate) to form a paste. The waterglass typically includes highly-caustic compounds such as LiOH, NaOH, KOH, or CsOH in an appropriate amount of water into which amorphous silica is dissolved. Additional amorphous silica is often utilized, which may be in the form of dry particles and/or a liquid form, such as a dispersion. During the formation of the geopolymer, a three-part chemical reaction takes place: 1) dissolving the aluminosilicate and additional amorphous silica into the waterglass, 2) polycondensation or polymerization of AlO4− and SiO4 tetrahedra into a random network; and 3) precipitation into circular polysialates.
Geopolymers are suitable for use in a variety of applications, including in coatings, refractory adhesives, low-CO2-producing cements, isochemical ceramics, and more. They are strong, light-weight, and quick setting, and are generally considered more “green” than other materials used in the art due to the lack of volatile organic compounds and the fact that geopolymers only release small amounts of CO2 compared to Ordinary Portland Cement (OPC). The production of OPC follows the reaction below [1, 2]:
This reaction emits CO2 in two ways: burning of the fossil fuel to provide the heat necessary for the reaction and as a direct reaction product. Whereas, the only CO2 emitted in the production of geopolymers solely comes from the burning of fossil fuels to calcine the kaolin into metakaolin. Producing 1 ton of OPC generates 1 ton of CO2, whereas, 1 ton of geopolymeric cement generates 0.180 tons of CO2.
Coatings are used for a variety of protective and decorative functions. Coatings may, for example, be used for protection of vehicles, structures, or their component parts, from corrosion, chemical degradation, temperature, pressure, radiation, abrasion, and weathering elements such as ice, wind, and rain.
Organic coatings have also been used for the purposes described above. Production of such coatings, however, often requires the use of harmful or hazardous materials. Some of the materials are volatile and enter the atmosphere during the coating production process or afterward, when the coating is in use. These volatile components are essentially pollutants and the adverse impact of these components on the atmosphere and environment renders them undesirable. Further, production of organic coatings often entails the use of large volumes of petroleum products, thus rendering the environmental footprint of these coatings even larger than from the volatile components alone. Organic coatings also tend to degrade or be otherwise damaged by high-heat conditions. Many organic bonds begin to decompose at temperatures around 400° C. or lower. Some organic compounds begin to breakdown or outgas volatile components at an even lower temperature.
Epoxy polymers have been in use since the 1940s. For ambient cure applications, epoxy resins are crosslinked with a variety of curing agents. Traditional epoxy coatings are solvent borne, and more recently 80-100% solids by volume. In the last two decades, waterborne epoxy systems have been developed, which can reduce the volatile organic compound (VOC) content due to the use of water as an exempt solvent. Epoxy coatings are desirable because of their high hardness, toughness, corrosion resistance, and adhesion. Waterborne epoxies typically have high hardness, toughness, and adhesion, but their corrosion resistance and water resistance is not as good as solventborne or high solids epoxies, and they typically have lower volume solids. Waterborne epoxies also lose adhesion after long-term immersion in 140° F. deionized water. For these reasons, waterborne epoxies have not been successful in penetrating the industrial protective coatings market for severe environments, such as immersion service coatings, tank linings, or coastal marine applications.
Inorganic coatings have a number of advantages over organic coatings. Inorganic coatings tend to be less expensive than organic coatings because they can be made from abundant natural resources. Inorganic coatings are also generally more highly resistant to heat than organic coatings. Traditional inorganic coatings do suffer from disadvantages as well, however. For example, traditional cementitious inorganic coatings tend to be brittle and crack easily, do not exhibit the same degree of flexibility generally found in an organic coating, and tend to adhere poorly to organic or polymeric substrates.
Geopolymer coatings, which are inorganic, also suffer from some of the disadvantages described above. For example, geopolymer coatings may suffer from shrinkage and cracking at high water levels, which may result in loss of adhesion or premature corrosion. This limit on the amount of water that can be used with traditional geopolymer coatings also limits the properties of the resulting coating. Literature generally provides the following ideal molar ratios for geopolymer components: 1.00 M2O; 1.00 Al2O3; 4.00 SiO2; and 11.00 H2O [4]. When the molar ratio of water is increased from the value given here, geopolymer coatings tend to shrink, crack, or the like.
The combination of a geopolymer and latex in a coating has been found to prevent cracking in a 2008 patent [3]. This patent is for a geopolymer composition involving geopolymer-containing filler particles and film forming geopolymer precursors. Both in situ and premade geopolymers are contained in the composition. Latex could be added as a toughening agent to the composition.
Geopolymers have been described as a possible filler for a curable epoxy resin composition for use as an electrically insulating material [5]. In this instance, the geopolymer portion is not created in situ, but rather used as a premade filler.
It has been previously reported that geopolymer and epoxy hybrid compositions can be created in situ [6]. A separate geopolymer paste was prepared and added to a mixture of liquid epoxy resin and curing agent. This mixture was cured at 60° C. for 6 hours then post cured at 180° C. for 2 hours. It was determined that when geopolymer is incorporated into an epoxy system, the thermal stability is improved. There are five components used to make this composition, and three separate mixing steps. These five separate components must be mixed at the time of use. In the first step, the three components of the geopolymer paste are mixed in one container. In the second step, the liquid epoxy resin and curing agent are mixed in a second container. At this point, the mixtures made in step one and step two are not shelf stable, and therefore cannot be stored over time. In the third step, the previous two mixtures are blended together.